Polycrystalline diamond (PCD) material comprises a mass of inter-grown diamond grains and interstices between the diamond grains. PCD may be made by subjecting an aggregated mass of diamond grains to a high pressure and temperature in the presence of a sintering aid such as cobalt, which may promote the inter-growth of diamond grains. The sintering aid may also be referred to as a catalyst material for diamond. Interstices within the sintered PCD material may be wholly or partially filled with residual catalyst material. PCD may be formed on a cobalt-cemented tungsten carbide substrate, which may provide a source of cobalt catalyst material for sintering the PCD.
PCD material may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. For example, tool inserts comprising PCD material are widely used in drill bits used for boring into the earth in the oil and gas drilling industry. In many of these applications, the temperature of the PCD material may become elevated as it engages rock or other workpiece or body with high energy. The working life of tool inserts may be limited by fracture of the superhard material, including by spalling and chipping.
U.S. Pat. No. 4,311,490 discloses a diamond layer comprising diamond grains and having at least two layers, one layer of coarser grains adjacent to a carbide mass and another layer of finer grains adjacent the coarser layer.
U.S. Pat. No. 5,135,061 discloses a cutting element including a cutting table that may comprise at least two distinct layers of superhard material bonded together, the front layer comprising a less wear resistant form of superhard material than the second layer.
European patent No. 1 330 323 discloses an abrasive composite compact comprising a sintered particle layer bonded to a cemented carbide substrate, the abrasive particle layer having three regions containing particles having different size distributions.
U.S. Pat. No. 6,446,740 discloses a cutting element having a cutting table which is formed from segments of an ultra hard material. Preferably, some of the segments are made from finer grade of ultra hard material while the remaining segments are made from a coarser grade of ultra hard material.
U.S. Pat. No. 7,694,757 discloses a cutting element including a TSP material layer over a substrate. TSP material is formed by “leaching” the cobalt from the diamond lattice structure of at least a portion of the PCD, or by forming PCD with a thermally compatible silicon carbide binder instead of cobalt. The TSP layer may include adjacent first, second and third layers, which may have different average diamond grain sizes and different porosities.
U.S. patent application publication No. 2008/0142267 discloses a PCD cutting element, which in operation (and as it wears to a worn condition) presents at least two cutting lips to the material being cut. At least part of the end working surface and at least part of the peripheral working surface are substantially free of catalyzing material. The PCD element may incorporate regions of different abrasion resistance, for example arranged in a series of layers, or in a series of concentric rings.
PCT publication No. WO2007/089590 discloses a shear cutter with functionally designed PCD composite overlay. The tool body can include a plurality of polycrystalline abrasive layers and a plurality of arresting layers. In this application, “arresting” refers to a property and/or effect of a material whereby cracks or other destructive or deteriorating occurrences within one area of material is halted, slowed, reduced, and/or isolated. The layer selections and the interface design amongst the layers can also preferably minimise thermal mismatch stress.
There is a need for a superhard cutter element having improved fracture resistance.